Field of the Invention
The invention relates in general to a circuit system, and more particularly to a technology that adaptively provides a supply voltage in a circuit system.
Description of the Related Art
A voltage-controlled oscillator (VCO) is an essential element commonly applied in various kinds of electronic systems. In general, the temperature of an electronic product rises after a period using the electronic product. The rise in the ambient temperature results a drop in the amplitude of output signals of the VCO, leading to degradation in the phase noise performance. On the other hand, oscillation start-up of a VCO can be more difficult in higher temperatures. For the above issues, in a current solution, the amount of a source current provided to the VCO is adaptively changed according to the ambient temperature. More specifically, the amount of a source current provided to the VCO is increased as the ambient temperature rises, so as to compensate the amplitude of output signals of the VCO.
FIG. 1 shows a schematic diagram of a known technology. An operating power of the VCO 120 is provided by a current source 140. Generally known to one skilled in the technical field, the current source 140 is a proportional to absolute temperature (PTAT) current source, which outputs an output current IPTAT having a size that is proportional to the absolute temperature. A drawback of the approach in FIG. 1 is that, to minimize the phase noise, a p-type metal-oxide semiconductor field-effect transistor (MOSFET) forming a current mirror in the current source 140 is designed to have a large size, which occupies a considerable chip area.
FIG. 2 shows a schematic diagram of another known technology. An operating power of a VCO 220 is provided by a programmable current source 242, and a control circuit 244 determines the amount of current to be outputted by the programmable current source 242 according to ambient temperature information detected by a thermal sensor 246. In practice, the control circuit 244 is usually built-in with a look-up table storing the corresponding relationship between the temperature and the amount of current. One drawback of the approach in FIG. 2 is that the thermal sensor 246 needs to be additionally provided. Another drawback of the approach in FIG. 2 is that, when an internal switch of the programmable current source 242 is switched to adjust the amount of current in response to a looked-up result, a discontinuous change in the source current may lead to jittering in the output frequency of the VCO 220. For a frequency division duplexing (FDD) system that highly demands a stable connection or a television tuner that requires a stable display image, frequency jittering is a severe issue.
FIG. 3 shows a schematic diagram of yet another known technology. An operating power of a VCO 320 is provided by a programmable current source 342, and a control circuit 344 determines the amount of current to be outputted by the programmable current source 342 according to ambient temperature information detected by an amplitude detector 346. With such approach, one drawback is that the amplitude detector 346 needs to be additionally provided, and another drawback is that frequency jittering may also be caused.
As a common feature of the three known technologies above, the amount of a source current is adaptively provided to a VCO according to an ambient temperature. However, the three known technologies also suffer from certain drawbacks.